This application incorporates by reference, and claims priority to, and the benefit of, German patent application serial number 19815132.2, which was filed on Apr. 3, 1998, and German patent application serial number 19914472.9, which was filed on Mar. 30, 1999.
The invention relates to an article of footwear that provides a dual energy management system to improve the biomechanical properties of the article of footwear.
During each footfall in walking, running and jumping, forces are acting between the ground and the foot. These forces are usually referred to as ground reaction forces (GRF). They can be quantified using appropriate measuring devices. The order of magnitude of the GRF for walking is typically 1 to 1.5 times an athlete""s body weight (BW). In running, the forces are typically between 2 to 3 times BW, and in jumping the forces are typically between 5 and 10 times BW. To compensate for the forces occurring during walking, running and jumping, complex movements take place in the human body, which lead to considerable stress on the anatomy.
If an average runner runs a distance of approximately 1 kilometer, his feet contact the ground approximately 1,000 times. This leads to a considerable accumulation of arising impacts, i.e., resulting shocks. For example, a person with a body weight of approximately 68 kilograms who runs a distance of approximately 32 kilometers per week is exposed to a force equivalent to approximately 4 to 6 million kilograms per week. For an athlete to sustain such a stress without any damage it is necessary for the body to compensate for, or absorb, the shock in a harmless way.
To this end, the human body is provided with a number of natural mechanisms. A portion of the shock is already absorbed by the moving parts of the body consisting essentially of bones, muscles, and cartilage. Furthermore, the human foot comprises cushioning consisting of fat and cartilage for damping the impact and reducing the stress. In addition, the degree of bending of the knee can play an important role in the influence of the impact forces during running. For example, it has been shown that the vertical force impact peak (VFIP), i.e., the force that is measured when the heel contacts the ground, is reduced by an increase in the degree of bending of the knee during running. Experiments confirm that the VFIP values occurring in persons who contact the ground first with the midfoot or forefoot area are negligible. The VFIP A values depend heavily on boundary conditions, such as, the speed of running and the hardness of the ground.
For a long time, consideration has been given to protecting the human body against impacts by providing footwear with cushioning. If footwear cushioning is to be optimized for specific kinds of sports, it is helpful to obtain information about GRF, i.e., the forces occurring when the foot contacts the ground. GRF designates all forces which act upon the foot or the body during contact with the ground. Furthermore, it is helpful to consider the time dependence of the forces acting on the foot or body.
The force-time pattern for each foot-ground interaction typically shows two distinct phases; an impact phase when the foot collides with the ground followed by a push-off phase when the athlete is propelled forward and upwards. FIG. 1a shows the landing motion of the foot in long distance running. About 80% of all runners contact the ground with the heel first. FIG. 1b shows the following push-off of the midfoot and forefoot. The corresponding vertical component of the GRF is shown in FIG. 1c. As can be seen, the curve consists of two distinct force maxima. The first maximum, corresponding to point P in FIG. 1c, occurs after 20 to 40 milliseconds (ms) as a result of heel impact. This value P was designated above as the vertical force impact peak (VFIP). Sometimes this peak value P is also called the xe2x80x9cpassive peak valuexe2x80x9d because during this short time interval the human body can not react and adjust to it. The second maximum, corresponding to point A in FIG. 1c, occurs after 80-100 ms and is caused by the push-off action of the midfoot or forefoot from the ground during running to move the runner forward and upwards for the next step. This peak value A is called the xe2x80x9cactive peak valuexe2x80x9d or the xe2x80x9cpropulsion peak value.xe2x80x9d
Studies have shown that the relative height of the passive and active peak values can vary with respect to each other depending on; the kind of sport, speed of running, anatomical formation of the feet, etc. In some cases, the values shown in FIG. 1c can change such that the active peak value has the same height as the passive peak value or even higher. It is, however, typical that two peak values occur which are separated by approximately 60 milliseconds.
The two types of forces have different consequences with respect to the human musculoskeletal system. Impact forces do not contribute to athletic performance. Impact forces, however, have been associated in a number of studies with chronic and degenerative injuries in various sports, especially, when the heel is involved. The goal, therefore, is to reduce impact forces under the heel using appropriate footwear sole constructions. The desired systems are the ones that deform easily under load and dissipate energy.
Magnitude and duration of active forces determine athletic performance, i.e., running speed and jumping height. This means, if an athlete wants to run at a certain speed, the appropriate level of active forces must be maintained. Thus, the intention is to enhance these forces with a footwear sole that minimizes energy dissipation as much as possible, and at the same time provides the necessary cushioning.
With respect to cushioning systems in footwear, and to deal with the undesired results of the forces, as discussed above, and to use these forces advantageously, the following approaches were used in the prior art. In U.S. Pat. No. 5,695,850, the concept is known to provide a sports shoe with a sole unit that is said to improve the performance of the shoe. This is to be achieved by using components of the shoe or the sole which xe2x80x9cregainxe2x80x9d the energy during running and transform it during the push-off phase from the ground, i.e., in the area of the active peak value in FIG. 1c, into a forward movement. To this end, the use of elastic materials either in the complete sole area or limited to the forefoot area is disclosed. Suitable elastic materials are, among others, 1,4-polybutadiene/rubber compounds or, as an inlay for the shoe, a mixture of ethylene vinyl acetate (EVA) and natural rubber.
German patent no. DE 87 09 757 discloses a sole unit consisting of an outsole and a midsole mounted thereon. The midsole is formed by a comparatively narrow frame-like extending strip defining a seat that is downwards closed by the outsole. Inside the seat two sole parts are provided, one of which extends from the forefoot part of the shoe to the beginning of the heel part where the second sole part is provided. The first sole part consists preferably of a plastic supporting inlay being comparatively yielding under pressure so that during walking with such a shoe a foot bed can be formed on the sole part providing a certain level of comfort. The sole part arranged in the heel area provides a shock absorber and consists of impact or shock absorbing material, for example, silicon.
U.S. Pat. No. 4,108,886 also describes the use of shock absorbing inlays in the heel part of a sole unit. U.S. Pat. No. 4,316,335 discloses the use of a shock absorbing material not only in the forefoot part of a sole, but also in the heel part, wherein, the damping properties of the heel part are better than in the forefoot part.
European patent no. 0 272 082, discloses the use of a spring plate in the forefoot area of a sole unit. The spring plate is used to take up energy during each step and to release the energy during the push-off phase.
All of the above described known concepts have the disadvantage that the suggested materials and material parameters for the heel or forefoot area are not adjusted or optimized for the time dependence of the above described passive and active peak values. Furthermore, the suggested materials are not coordinated with the other materials used in the sole so possible additional effects are not taken into account. Therefore, the intended effect is only partly achieved and during running a xe2x80x9cspongyxe2x80x9d or xe2x80x9cspringyxe2x80x9d feeling arises, which considerably hinders forward movement.
It is an object of the invention to provide a well-balanced sole unit where the passive and active force peak values arising during the natural course of motion are optimally taken into account, and the natural dynamics of the movement are optimally used. It is another object of the invention to provide a sole unit having a low cost and long durability.
In general, the solution of the above problem is obtained by a sole unit for an article of footwear, comprising at least one sole layer. According to the invention, this sole unit is divided from front to rear, i.e., the horizontal direction, into at least two different parts. The first horizontal part extends over the forefoot portion of the article of footwear. The forefoot portion can include either the forefoot area of the sole or the forefoot and midfoot areas of the sole. The second horizontal part extends over the rearfoot portion.
The present invention includes a unique feature, which provides in the forefoot portion of the sole unit a layer of material having a predominantly elastic damping characteristic. Such a material has, in a forward movement, the characteristic that the pushing-off from the ground is supported by the xe2x80x9celastical back scatteringxe2x80x9d of the kinetic energy, i.e., the conversion of the elastic potential energy stored in the elastic material to kinetic energy. In contrast, the rearfoot portion of the sole unit (the heel part), preferably includes a material layer having a predominantly viscous damping characteristic. The use of a viscous material leads to repulse-free absorption of the impacts occurring during running, in particular on the heel of the foot, since the energy of the impact is transformed into heat.
The elastic and viscous materials used according to the invention are characterized by their material specific energy loss. The inventors have discovered that the critical material parameter of optimal materials for the rearfoot area and the forefoot area is the loss of energy. The energy loss is the parameter obtained from the response of a test material exposed to a force, and is to be determined experimentally.
To determine the response in a biomechanically adjusted manner, a procedure is used where a sample of the material to be tested is subjected to a dynamic force corresponding to the force acting upon the feet during human running. Preferably, a GRF-force profile like that shown in FIG. 1c (separately for the forefoot and rearfoot area) acts upon the test material. A certain energy is fed into the material leading to a deformation of the body of the material. This deformation is the result of the material""s specific elastic properties having a certain time dependence and thereby leads to a recuperation of the energy, i.e., the energy fed into the material is returned by the springing back of the material. The energy recuperated in this way is for physical reasons always less than the fed energy, since a part of it is, depending on the material, transformed into heat. If the recuperated energy is subtracted from the fed energy, a positive difference is obtained which can be designated as xe2x80x9closs of energy.xe2x80x9d
According to the invention, it has been shown that elastic materials suitable for the forefoot portion should have an energy loss not exceeding about 30%, preferably not exceeding about 27%, and more preferably not exceeding about 24.5%, which leads during the push-off phase of the foot to a measurable support for the upward and forward movement of the foot. Furthermore, it has been shown that it is beneficial for the viscous material used according to the invention for shock absorbing in the rearfoot portion, to have an energy loss exceeding about 50%, preferably exceeding about 55%, and more preferably exceeding about 60% to lead to a measurable reduction of the risk of injury. Finally, it has been shown according to the invention that by a combination of elastic and viscous materials in the forefoot and rearfoot portions, respectively, which have a difference in loss of energy exceeding about 20%, preferably exceeding about 28%, and more preferably exceeding about 35.5%, a combined effect is obtained leading to the improved performance of the athlete, i.e., the running or walking takes place with reduced energy consumption. This was experimentally determined by comparative studies of the oxygen consumption of athletes.
In one aspect, the invention relates to an article of footwear including a forefoot portion, a rearfoot portion, and a sole. The sole includes a first area which extends at least partially over the forefoot portion and a second area which extends at least partially over the rearfoot portion. The first area is constructed of a predominantly elastic material having a material specific loss of energy that does not exceed about 30%, preferably not exceeding about 27%, and more preferably not exceeding about 24.5%.
In another aspect, the invention relates to an article of footwear including a forefoot portion, a rearfoot portion, and a sole. The sole includes a first area which extends at least partially over the forefoot portion and a second area which extends at least partially over the rearfoot portion. The second area is constructed of a predominantly viscous material having a material specific loss of energy that exceeds about 50%, preferably exceeds about 55%, and more preferably exceeds about 60%
In yet another aspect, the invention relates to an article of footwear including a forefoot portion, a rearfoot portion, and a sole. The sole includes a first area which extends at least partially over the forefoot portion and a second area which extends at least partially over the rearfoot potion. The first area is constructed of a predominantly elastic material having a material specific loss of energy. The second area is constructed of a predominantly viscous material having a material specific loss of energy. In this aspect, the difference between the loss of energy in the first area and the loss of energy in the second area exceeds about 20%, preferably exceeds about 28%, and more preferably exceeds about 35.5%.
Embodiments according to the foregoing aspects of the invention can include the following features. The first and second areas of the sole unit can be disposed in the same layer of the sole unit or in two different layers of the sole unit. The sole unit can include at least one additional material layer, for example, an insole or an outsole. If additional layers are used, it is necessary to take additional material parameters into consideration, for example, the dynamic stiffness of both the elastic and viscous material in comparison to the dynamic stiffness of the material(s) that form the additional layers. The dynamic stiffness is the slope of the curve in a force-deformation diagram, like that shown in FIGS. 3 and 4, between certain force intervals; for example, between 1000N-1,5000N and between 200N-400N.
Taking the dynamic stiffness into account in embodiments where the sole unit consists of several layers is important, since the elastic properties in the first area and the viscous properties in the second area do not take effect if the wrong materials are chosen. The situation is as in a series of two coupled springs. The effect of spring 1 with a specially adapted spring characteristic does not take effect, if the spring constant of the second spring is smaller than the spring constant of the first one. In this case the damping characteristic of the coupled springs is predominantly determined by spring 2. Only after spring 2 has been completely compressed, does spring 1 becomes effective.
In an embodiment of the invention including a sole unit with additional layer(s), the additional layer(s) preferably comprises material(s) with a dynamic stiffness equal to or greater than the dynamic stiffness of the viscous and elastic materials. For the viscous material, this is particularly relevant for forces between 200N and 400N.
Suitable elastic synthetic materials for constructing the sole first area in accordance with the invention can comprise a combination of ethylene vinyl acetate (EVA) and natural rubber. For example, the elastic synthetic materials can comprise 50% ethylene vinyl acetate (EVA) and 50% natural rubber. Suitable viscous materials for constructing the sole second area in accordance with the invention can comprise a butyl-polymer. These synthetic materials are particularly suited as materials for a sole unit for the dual energy management system according to the invention.
These and other objects, along with advantages and features of the present invention herein disclosed, will become apparent through reference to the following description of embodiments of the invention, the accompanying drawings, and the claims.